Adaptive microstrip array using parasitic elements

ABSTRACT

A plurality of strip-line elements are used as an antenna array. The  direvity of the array is controlled by controlling the terminating reactance of the closely-coupled stripline elements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to field of electronic communication. Moreparticularly, the invention pertains to antenna design. By way offurther characterization, this invention pertains to an electronicallysteerable antenna. By way of further illustration but without limitationthereto, this invention will be described as it pertains to an antennauseful in missile applications.

2. Description of the Prior Art

It is known to use adaptive array antenna arrays to reduce externalinterference reaching a receiver by shaping the antenna pattern to steernulls, or minima of sensitivity, in the directions of the interference.This general technique has been in existence for some time. All knownadaptive arrays, to date, use weighting on every element followed by anelectronic summing network which combines the signal from each of theantenna elements electronically. That is, the signal received by eachantenna element is weighted in amplitude and/or phase by means ofelectronically controlled phase shifters, attenuators, amplifiers, etc.These weighted signals are then summed together to form an array output.This array output is monitored continuously and the weights at eachantenna are changed to minimize interference power in the receivedsignal.

Although satisfactory for many purposes, adaptive arrays of the knowntype encounter problems when the number of elements of the array arespaced closely to each other, less than a one-quarter of a wavelength,due to interaction between the elements. This interaction causes thepattern to vary sensitivity in response to the weight values which inturn limits the depth, width, and the pointing accuracy of the patternnulls. Such failings have made adaptive array techniques of little valuein compact arrangements such as required in the airborne missile arts.Another unfavorable characteristic of the prior art adaptive arrays isthe complexity and cost of the weighting circuitry and its correlationloop. Further, this type of construction is complicated by havingcircuitry for those adaptive arrays that operate at an intermediatefrequency. For such arrays an expensive mixing stage is required foreach weighted element.

SUMMARY OF THE INVENTION

The invention employs a plurality of antenna elements arranged in astandard array pattern. One of the elements of the pattern is connecteddirectly to the utilization device which, in the illustration examplesis a receiver. The remaining elements are each reactively weighted suchthat the resonance and coupling characteristics of each element may bealtered. Each element that is not connected to the receiver is parasiticto the element which is connected to the receiver such that itscontribution to the received signal is reflected to the elementconnected to the receiver. The weighting of each parasitic element isdetermined by a control circuit which receives an output from thereceiver such that constructive or destructive interference can occurbetween the reflected waves leading to lobes or nulls in the pattern ofthe sensitivity of the antenna. Such directivity and sensitivity curvesare well understood in the antenna design art and will not be discussedin greater detail herein.

It is accordingly an object of this invention to provide an improvedreceiving system.

A further object of this invention is to provide a light weight,low-cost, steerable antenna array.

Another object of this invention is to provide a reactively steeredadaptable array utilizing a minimum of electronic components.

A still further object of this invention is to provide a reactivelysteered antenna array which may be configured in a microstrip antenna.

These and other objects of the invention will become apparent to thoseversed in the art and considering the appended description, claims anddrawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an environmental view showing the invention in an operationalenvironment;

FIG. 2 is a plan view of the antenna according to the invention;

FIG. 3 is a partial sectional view taken along lines 33 with anassociated block diagram; and

FIG. 4 is a block diagram of the control circuit shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an aircraft 11 shown in communication with amissile 12 which is headed toward a target 13. Missile 12 has a patchmicrostrip antenna illustrated at 14. Antenna 14 is in electroniccommunication with an antenna 15 on aircraft 11. As illustrated,antennas 14 and 15 are electronically steered such that the directivitycurves provide for optimum transfer of communication withoutinterference.

Referring to FIG. 2, antenna 14 is shown in plan view and has a centralelement 16 and a plurality of parasitic elements 17 spaced therefrom.Elements 16 and 17 are on a dielectric support 18 and have connectionpoints 19 on each element where electrical connection is made thereto.It will be observed that connection points 19 are asymmetric withrespect to antenna elements 16 and 17. This asymmetry is to matchimpedances between the transmission line of associated circuitry. As iswell understood this placement may vary if the characteristic impedanceof the transmission line differs from the seventy-two ohm configuration,illustrated.

Referring to FIG. 3, it may be seen that on the opposite side ofdielectric sheet 18 a ground plane 21 is positioned a form the groundplane of a microstrip assembly. Such microstrips are sometimes termedstriplines. Ground plane 21 has a plurality of apertures 22 permittingconnector 23 and 24 to be attached to elements 17 and 16 respectively.Connectors 23 connect antenna elements 17 to a phase shifter 25. Phaseshifter 25 may be a varactor diode which is a two element electronicdevice which changes its inductive impedance in response to an appliedd.c. voltage. Such devices are well understood in the electronic artsand choice between appropriate ones is made with routine skill byelectrical engineers conversant with such circuit design. Element 16 isconnected to a receiver 26 which is a conventional RF receiver having anelectronic output. The output of receiver 26 goes to other utilizationdevices but also feeds a plurality of control circuits 27. Controlscircuits 27 are each associated with a phase shifter 25 to control theimpedance thereof.

Referring to FIG. 4, the details of control circuits 27 are illustrated.An input from receiver 26 is connected to a synchronous detector 31which has an output which is connected to a multiplier 32 which has itsoutput connected to an integrator 33. An audio oscillator 34 ofconventional design feeds its output to synchronous detector 31 and toan attenuator pad 35. The purpose of attenuator pad 35 is to reduce thevoltage to an operational level corresponding to the output ofintegrator 33 and the output of attenuator 35 and integrator 33 aresummed in a summing circuit 36 which may be an op amp and the analogoutput therefrom is connected to elements 17.

Audio oscillator 34 provides signal to synchronous detector 31 aspreviously described.

The antenna pattern for the array is formed in accordance to the valuesof the load impedance determined by the terminating phase shifters 25.Thus, the incident radiation reflected from the various parasiticelements 17 to central element 16 is controlled by the value of the loadimpedance of phase shifters 25. Receiver 26 sensitive to the totalcurrent induced on central antenna element 17 which is produced by thesum of the reflected waves and hence constructive or destructiveinterference occurs between the reflected waves leading to the lobes andnulls of the antenna directivity curves as are well understood inradiation transfer arts.

Control of the antenna pattern is achieved by the components shown inFIG. 4. Audio oscillator 34 has two functions. It causes the value ofthe terminating reactance to have a small alternating componentimpressed upon it which, in turn, produces a corresponding alternatingcomponent in the receiver output. This alternating component serves as areference signal for the synchronous detector 31 and for variations ofrelatively small magnitude the output of synchronous detector 31 isproportional to the ratio of the change of the output of audiooscillator 34 and the variation in the output from receiver 26.

This output from synch detector 31 is multiplied by a constant which isnegative if minimization of the incident output is desired or ispositive if maximization is desired. The output from multiplier 32 is anintegrated by integrator 33 to produce an error voltage that is used tocontrol the terminating reactants and requires a control circuit 27 tobe associated therewith. In the described example, a five element arrayhas been shown. When the array is tuned for a center of 4.0 GHz theillustrated circuit has the ability to steer a null in the patterntoward the source of intereference that has depth of 35 dB relative tothe pattern lobes and an angular width of 25 degrees. Such a null hasbeen able to cancel an interference signal as might originate fromtarget 13 or other spurious radiation. A similar gain in the antenna canbe steered if the positive multiplier is utilized. The afore describedadaptive pattern control has permitted 360 degree steering in closelyspaced microstrip antennas and has produced substantial benefits overdesigns previously used.

What is claimed is:
 1. A reactively steered adaptive antenna arraycomprising:a plurality of resonant elements arranged in a predeterminedpattern; a utilization device connected to a selected one of saidresonant elements having an electrical output; a plurality of reactivelytuneable elements equal in number to the remaining number of resonantelements and connected thereto one reactively tuneable element to eachresonant element; and a plurality of control circuits each including asynchronous detector and equal in number to the number of reactivelytuneable elements and connected thereto and each control circuit havingits synchronous detector connected to the utilization device to receivethe electrical output therefrom, whereby the reactively tuneable elementcauses the resonant element connected to the utilization device signalsin a phase controlled manner to effectively steer the directionalresponse of said pattern of resonant elements.